MOST COOKS would be insulted if you told them their food gave you heartburn. Not Jews. In Jewish cuisine indigestion is a compliment. The tradition is that a meal needn't taste great, just so long as you feel full afterwards. Perhaps Jews have a similar approach to literature, seeking to finish a book like they finish dinner: staggering for a chair, gasping: 'That's it, I can't take anymore.' But in Roots Schmoots Howard Jacobson has served up a literary Lean Cuisine. It doesn't taste low calorie - on the contrary, it tastes great, there's a delicacy on every page. But in the end you still feel hungry. If Jacobson was a kosher restaurant, he'd have his licence removed.

The book is an account of his travels in America, Israel and Lithuania: a 'Jewjewjourney'. While Alex Haley was a time traveller, searching into the past for his black roots, Jacobson is a globe-trotter, journeying among today's Jewish communities looking for the source of his identity. He meets Jews of every type: aggressive yet lachrymose Jews in upstate New York's Borscht Belt; medieval yet warm ultra-Orthodox Jews in Brooklyn; Jewish yet Christian Jews, who recognise Jesus, in Los Angeles. He also finds fanatically right-wing Jews in West Jerusalem, committed left- wing Jews in East Jerusalem, and Israeli Jews who don't know they're Jews in the Red Sea resort of Eilat. Finally, there are the shrunken, haunted, despised Jews of the Old Country, Lithuania.

Jacobson meets them, hears their stories and describes them artfully. When he meets a Nazi-hunter in California, he notes what he says as faithfully as a reporter, but observes him like a novelist: 'He has dark eyebrows which contradict each other. One is shaped comically into an interrogation mark. The other is an underscoring, heavy and final . . . His lips make a wet sound whenever they meet, so that his sentences are punctuated by a sort of moist percussion.'

He has sympathy for all of them, even those he hates. He lambasts the Torah-thumpers of the Israeli right, who advocate the death-penalty for Jews who do not observe the sabbath and who see the churches of the Holy Land as blasphemous blemishes fit only to be razed to the ground. Yet his heart sides with them when, that same evening, he meets a Gentile do- gooder from Tunbridge Wells, whose anti-Zionism has become anti-Semitism, making him one of the author's 'enemies of the soul'. In other words, Howard Jacobson has discovered that when it comes to gut instincts, his Jewishness is more important to him than either his Britishness or his politics, and perhaps even his principles.

He makes another, related discovery. As he travels he becomes less apologetic, not about his Jewishness in the company of non-Jews - an angst he worked over thoroughly in his first novel, Coming from Behind - but about the much less discussed emotion of shame among one's own. Jacobson shrugs off the ethnic cringe he once felt at seeing his people engage in the crass, the gauche, and the plain grob. And he banishes the old feeling of inadequacy prompted by his ignorance of Hebrew and religious custom, and his lack of faith.

By journey's end, having seen as many Judaic forms as Jews, he concludes that his atheistic brand of Judaism - defined by the pursuit of free thought, a constant desire to debate and dispute, a willing immersion in art made by Jews, if not in 'Jewish art', and the consumption of Jewish food - is legitimate. No longer out- jewed by the black-hatted and side-curled ultras of Stamford Hill, he declares himself kosher.

He goes further, explaining to a woman at a dance for LA Jewish singles that the still-religious Jews are a kind of proletariat, stoking the fires of Judaism's engine room, while 'the real aristocracy of the Jewish faith are its intellectuals and non-observant philosophers'. Take a wild guess who Howard has in mind.

These two realisations (which, in truth seem more like confirmed prejudices) are linked. The elements that make up his secular Jewish identity - the warmth, the almost obsessive sense of past, the humour soaked in tragedy - also make up the tribal glue that bond him to his fellow Jew. Even the Jew for Jesus has some of it when, incredibly, he asks the author for a donation, because, 'In the end, we're still Jews and we have to help one another.'

But, and this is where the book founders, Jacobson has confused roots with fruits. Those characteristics he identifies in himself and celebrates in others have different meanings for him and them. The author places them at the very root of his Jewish identity. For the people he encounters, however, those traits - of disputatiousness, of hospitality, of rudeness - are merely by-products. They are the fruits of something more fundamental: a sense of faith, culture and community.

Jacobson no longer has any of these things. All he has is the detritus of a time when he did. He can still go a few rounds with theological dogmatists only because of his childhood years in Hebrew school. He can laugh at Jewish refugee humour only because he still has the smells of his grandparents' house, the aromas of Vilna, in his nostrils.

This is an untenable identity. It is parasitical on his past and the past of others. The people he meets all have a current Judaism in their lives, and the manifestations of it spring from that. Howard Jacobson shows the same effects they do, but lacks their cause. He's all schmoots and no roots.

Which is why he can be buffeted all over the place, in a tirade against Jewish supremacism at one moment, confessing to sheer prejudice and distrust of the Arabs the next. With only a Woody-Allen-and-bagels Judaism to guide him, the only consistency he can achieve is emotional.

The form of the book suffers accordingly. It reads like an unedited diary. Jacobson's skill with the one-liner - 'Multitudinous children, uncertain whether they are themselves or their siblings', 'a rancid boy in shoes so filthy he must have sent them away to be filthied' - means it's always entertaining, but it is flabby. Amos Oz's In The Land of Israel is still the best Jewish travelogue because the Israeli's left-Zionist worldview lent it discipline. Jacobson's structure is built not by ideas or argument, but by geography and an airline timetable.

The clues for the leaner, tauter book Jacobson could have written are to be found in those passages, buried in all the reportage and anecdote, which show the author stepping back. These contain flashes of brilliant insight: his observation that Jesus's most non-Jewish act was to overturn the trading temples in the Temple, since Jews, never ascetics, 'believe there is no distinction between the world's business and the business of the spirit'; or his claim that Gerald Ratner's 'crap' remark was a peculiarly Jewish act of self-hatred: 'For whosoever makes or sells what he knows to be crap, what he wouldn't want for himself, admits his self-hate.'

I suspect that Roots Schmoots will not be the last we hear from Howard Jacobson on this subject. For one thing, there is a drama in this book, unspoken, but present throughout, which one senses is yet to be resolved. The author refers frequently to, and quotes from the diary of, his wife. She is Australian, and a Catholic. This is the issue Jacobson didn't dare touch, and yet it it goes right to the root of the roots he's unearthed. For children of marriages like his, where the mother is not Jewish, are not universally regarded as Jews. How does Howard Jacobson feel, knowing that he cannot pass on the culture which defines him? Is it perhaps that the secular Judaism that Jacobson personifies can never last beyond a generation? Might it not always require a traditional source to give it life?

Maybe this truly personal journey might make it as a future Jacobson offering, one in which Jacobson gets past the sauce, and onto the meat-and- potatoes substance. That, at least, should keep his kosher readers happy.